During crown gall tumorigenesis a specific segment of the Agrobacterium tumefaciens tumor-inducing (Ti) plasmid, called the T-DNA, integrates into plant nuclear DNA. Similar 23 base- pair (bp) direct repeats at each end of the T region signal the T- DNA borders. T-DNA transmission (transfer and integration) requires only the right-hand repeat in cis, but a 24 bp sequence, called overdrive, which normally flanks right repeats greatly stimulates the process. Although right border repeats function best in one orientation, overdose functions in either orientation. To understand the mechanism of T-DNA transmission, we will examine the molecular intermediates which form during T-DNA transmission and determine the roles overdrive and the border repeats play in their formation. Through genetic and biochemical experiments we will identify proteins which interact with these sequences. To understand how overdrive acts, we will examine the effects of its location on the ability to stimulate T- DNA border function. To examine the molecular mechanism by which T-DNA integrates into plant DNA, we will characterize changes that occur in host DNA during T-DNA integration. Our experiments will contribute to understanding the unique and important recombination events that occur between bacterial and host DNA during crown gall tumorigenesis, and they will improve the T-DNA as a model system for studying tumorigenesis, gene expression, and host-pathogen interactions. This research will identify factors required for tumor establishment and may suggest ways to block tumorigenesis and viral tumor induction in animals. Understanding the mechanism of T-DNA integration may also shed light on the mechanism of adenovirus integration, a process that may involve similar molecular events. Agrobacterium and rickettsiae (bacteria which cause trench fever, Q fever, and Rocky Mountain spotted fever in humans) share lifestyle similarities (close association with eukaryotic host cells and plasmids associated with virulence) in addition to a very close phylogenetic relationship. Thus, A. tumefaciens may serve as a model for rickettsial pathogenesis as well as human tumorigenesis.